Industry and safety standards require that liquid storage tanks, and particularly petroleum storage tanks of the type located on petroleum tank farms, be given periodic checks to determine the liquid level of each tank.
The state-of-the-art in liquid level indicators is comprised of an apparatus consisting basically of a metal tape; an in-line tube assembly; a set of wheel pulleys; a float assembly; a take-up reel; and a mechanical liquid level indicator. One end of the tape is connected to the float, while the other is connected to the take-up reel. The tape is transported between the two termination points by the wheel pulleys. Both the tape and wheel pulleys are enclosed within the in-line tube assembly consisting of a horizontal tube and two vertical tubes. One of the vertical tubes is attached to the top of the tank while the other is attached to a tank-external housing containing the take-up reel. The housing also includes a mechanism that in combination with the take-up reel and float allows the liquid level to be directly displayed and viewed through a window on the housing. Some of the current devices also have a receiver, hard-wired to the mechanical apparatus, to allow the liquid level to be monitored at a remote station. Some of the major problems inherent in the current apparatus include:
a. Prior to installing conduit for a remote receiver it is necessary that building and fire permits be obtained; that area plot maps be available; and that ground be distrubed to run the conduit. Additionally, when a large number of tanks are located on a large parcel of land the installation costs can be prohibitive.
b. The installation of liquid activated switches that when closed energize an alarm circuit may require, in cases where tank contains volatile liquid, that the tank be emptied and purged before installation can commence.
c. The existing apparatus is dependent on an external power source. Therefore, if there is a primary power failure the apparatus ceases to function.
d. Some current apparatus require that the signal generation and emitting elements come in contact with the liquid in the tank. If the liquid is corrosive it may be necessary that the elements be made or protected by special material.
e. Existing apparatus using telephone lines to transmit liquid level signals are subject to accidental line breakage and/or deterioration by exposure to the elements.
The solutions to the above listed problems are given in the Disclosure of Invention section.
The prior art searched did not disclose any patents or publications that were directly related to an automatic liquid level indicator and alarm system of the type disclosed herein. However, the following U.S. Patents were considered in the investigation and evaluation of the prior art relative to the existing apparatus used with the invention:
______________________________________ U.S. PAT. NO. INVENTOR ISSUED ______________________________________ 3,148,542 Clift 02/09/61 3,200,645 Levins 08/17/65 3,459,042 Brown 08/05/69 3,484,774 Borgnakke 12/16/69 3,646,293 Howard 02/29/72 ______________________________________
The Clift patent describes the apparatus that interfaces with the preferred, second and third embodiments of the invention. Clift describes a method for gauging the liquid level in a tank by means of mechanical apparatus. The apparatus involves a float having a tape attached thereto which extends over wheel pulleys to the tank exterior where visual indicia correlated to the liquid level height can be read directly at the tank. The Levins patent relates to electric position sensors of the variable resistance type that are packaged in a measurement probe used to measure the level of liquid in a tank. Brown describes a solid-state telemetering device that includes a float-operated sensing unit that senses changes in liquid level. Borgnakke discloses a magnetically actuated liquid level indicator consisting of a stand pipe containing magnetic multiple ball swimmers. Outside along the stand pipe is a series of magnetic electric switches that are closed by the swimmers and that are connected to indicators. The Howard patent also uses magnetic switches activated by magnets moving past the switches. The switches generate signals indicative of the movement of the magnets.